With the remarkable development of small, thin, and high-functionality portable electronic equipment such as mobile phones and PDAs, there is a growing demand for smaller, thinner, and high-capacity batteries as their power source. Lithium ion rechargeable batteries can be designed small and high-capacity, and in particular, flat prismatic types are suitable in making the equipment thinner; they have therefore been increasingly used as the repeatedly usable rechargeable battery for portable electronic equipment.
Because lithium ion rechargeable batteries have high energy density and contain a flammable organic solvent as electrolyte, it is essential to take account of safety measures. They must have such safety features as to ensure that no damage is caused to the equipment or injury to the user in the event that an abnormality arises for some reason. For example, if the positive and negative terminals of the battery are short-circuited for some reason, a large short-circuit current flows in high energy density batteries, whereupon the inner resistance generates Joule heat and the battery temperature rises. A temperature rise in the battery leads to a rapid increasing of gas pressure inside resulting from reactions between positive electrode active materials and electrolyte, or from evaporation or decomposition of electrolyte, which may cause fire or explosion of the battery. Batteries may fall into a high-temperature state not only because of external short-circuiting but also of overcharge; the same applies if the portable electronic equipment loaded with the battery is placed near a heater or left inside a car parked in a hot weather environment.
A battery abnormality can be induced by any of electrical, mechanical, or thermal factors; thus non-aqueous electrolyte batteries represented by lithium ion rechargeable batteries are provided with safety features for preventing batteries from falling into an abnormal state and for evading a further dangerous state even if an abnormality should arise. Such features are usually incorporated in batteries as their own natures; for example, active materials on the electrodes and electrolyte may be made not to be excessively reactive, or, a polyolefin porous film may be employed for the separator because of its “shutdown function,” in which minute pores are softened and close under an abnormally high temperature. Further, it is necessary to prevent the battery from exploding when the battery is exposed to a high temperature condition. Thus, the battery is provided with a safety vent for externally releasing abnormally increased internal pressure thereof. As preventive means for protecting batteries from damaging by the aforementioned short-circuit, relatively large-sized lithium ion rechargeable batteries are usually provided with a protective feature such as a Positive Temperature Coefficient (PTC) element connected in series to the input/output circuit at the sealing end, which limits current flow in the event of external short-circuiting. Small batteries that cannot include the PTC element inside are normally provided with a PTC element or temperature fuse as outside circuit components which are then integrally formed with the batteries. Further, a circuit for protecting the battery from overcharge and over discharge is an absolute requirement. In general, these constituent elements are all packed with the battery inside a pack case to form a battery pack.
However, battery packs using pack cases are not suited to portable electronic equipment that are re-modeled in short cycles, because the manufacturing cost of molding dies used in the resin molding of pack cases tends to be high, and the time required for designing new molding dies is relatively long. Thinner battery packs with resin-molded pack case as an outer case also have limitations in making portable electronic equipment smaller and thinner because of the limitations on the moldable thickness in the resin molding process.
Furthermore, in order to prevent the user from disassembling a battery pack for wrong use or for satisfying curiosity, it must have a design that is hardly disassemblable, or a design that alerts the user that it has been disassembled. Taking account that the battery packs are used for portable electronic equipment, they also need to have a rigid structure that can withstand vibration or shocks in a falling accident, and a moisture resistance, particularly for the electronic circuit parts. In achieving the structure having a disassemblablity, a certain rigidity, and a moisture resistant, the idea has emerged that a battery may be united with a circuit substrate including a battery protective circuit by resin molding.
The conventionally known resin-molded battery packs are disclosed in Japanese Laid-Open patent Publications Nos. 2002-134077 and 2002-166447, in which a battery and a circuit substrate are connected by a connecting member to form an intermediate product, which is placed inside a die, and resin is filled around the intermediate product such as to expose external terminals formed on the substrate to the outside, to unite the battery with the substrate.
Further, Japanese Laid-Open patent Publication No. 2000-315483 discloses a structure in which a battery and a circuit substrate are connected by a connecting member and placed inside a die, and the circuit substrate is resin-sealed and fixed on the battery or its pack case (battery lid), or both the circuit substrate and the battery are resin-sealed.
When constructing a battery pack by a resin molding in realizing the size reduction and the thickness reduction, an optimum construction is such that constituent elements including a circuit substrate are placed at the side of a sealing end of a rechargeable battery, and a resin is filled and molded between the sealing end of the rechargeable battery and the circuit substrate. However, there has been a problem that an externally releasing opening of a safety vent is filled with the molded resin, thereby the safety vent loses its function.
Furthermore, the battery pack is brought to be placed inside equipment where constituent elements are densely arranged, particularly in the case of equipment having a reduced size and thickness. In such configuration, the equipment may be damaged by electrolyte released together with gas when the safety vent operates. A desirable construction is such that when the safety vent operates, only gas component is externally released, and liquid component such as the electrolyte is not externally released. However, the safety vent opens its release opening at a burst when it is reached to a critical operating pressure. Accordingly, it is difficult to prevent the liquid component from released together with the gas component. In the battery pack where a conventional pack case having constituent elements including a rechargeable battery therein, it is possible to provide means for holding a released electrolyte to stay within the pack case. However, a battery pack using a resin mold has no case-like member, and so requires a construction for preventing a liquid component from being externally ejected.
It is an object of the present invention to provide a battery pack which is constructed by resin molding without compromising the explosion-proof features of the safety vent.